Gas cleaning method



@d 6,'1'942. X Q LONG ETAL i 2,298,139

' GAS CLEANING METHOD Filed Dec. l2, 1939 Patented r.6,19 4z GASCLEANING METHOD Carleton C. Long and George E. Deeley, Beaver,

Pa., assignors to -St.

Joseph Lead Company,

New York, N. Y., a. corporation of New York Application December 1 2,1939, Serial N0. 308,876 I -1.

6 Claims. (Cl. 'Z5-88) This invention relates to an improved method forremoving dust from gas by first conditioning the gas with steam and thenpassing the gas through a system of 4tubes so arranged as to condensethe steam and create a large degree of turbulence in the gas stream, andnally separating the water from the cleaned gas.

The invention is particularly applicable to the removal of zinc dust orblue powder from the carbon monoxide gas produced as a by-product in thereduction by carbon of zinc oxide materials.

The removal of zinc fume or blue powder from the gases poses aparticularly difcult problem well known to those versed in the art ofcontinuous zinc smelting processes. In such a process zinc oxidematerials, more ,or less intimately admixed with coke or othercarbonaceous matter, are heated to a temperature sufficiently high topromote the ready reduction of zinc oxide to zinc metal vapor.Concornitantly formed is carbon monoxide gas from the carbon enteringinto the reaction. The zinc vapor and carbon monoxide are led away fromthe hot charge to a suitable condenser Where a portion of the zinc vaporis condensed vto liquid zinc metal. The condenser may be of the surfacetype wherein contact of the zinc vapor with a relatively cool surfacebrings about condensation of the zinc, or, the condenser may be of theinternal type wherein contact of the zinc vapor with a mass of moltenmetal effectively condenses the zinc vapor to zinc metal. -In eithertype, a certain portion of zinc vapor remains .uncondensed, the minimumamount being a function of the temperature of the exit part of thecondenser.

The higher the 4condenser temperature, .the

and water sprays v bubbling through water.

' denser gas.

greater will be the portion of non-condensed 40 zinc vapor. For example,with an entrance gas composition by volume of zinc vapor and carbonmonoxide and other diatomic gases, this being representative of actualexperience, a condenser temperature of 500 C. under an absolute pressureof 383 mm.-Hg will permit a loss, as non-condensed zinc vapor, of about0.75% of the zinc vapor entering the condenser. At the condensertemperature of 550 C. the minimum amount of zinc remaining uncondensedis 1.83% of the total zinc entering the condenser, while at 600 C. theloss is 6.38% of the total zinc.

Inasmuchas 'the zinc condenser temperatures are frequently of the orderof 550-600 C., 55 of the above,

4approximating those the amount .of zinc vapor accompanying thecondenser 4exit gases, whilegnot great in comparison with 'the amount ofmoltenmetal recovered from the condenser, is of appreciable magnitudewith reference to future 'handling and utilization of the condenser gas.In order that this gas be handled by vacuum pumps, for example, it mustbe substantially dirt-free.

The rst step in cleaning the condenser exit gases is an abrupt coolingfrom temperatures in the condenser to a temperature not far removed fromnormal atmospheric or room temperature. In practice, this isaccomplished effectively by passing the gas through a curtain of watersprays, followed by Shock chilling of this type condensesv all the zincvapor to very fine zinc dust, or blue powder as it is frequently termed.A large portion of the blue powder is trapped by the water and washedaway with it, from which it may be recovered by settling ponds or othersuitable apparatus. A portion of the finest powder, however, escapesentrapment with the water andpasses on with the gas stream.

In actual practice, observations extending over a period of years haveshown that from to 95 per cent of the zinc vapor accompanying thecondenser exit gases is trapped or collected by passage through theaforementioned gas washer. In practical terms, this means that with acondenser handling 16 tonsl of zinc per day and operating at 570 C. at332 mm. Hg absolute pressure, the amount of zinc Vapor accompanying thecondenser exit gases is 1280 pounds per day. If 93% (or 1190 pounds) ofthis zinc is removed by the washer, some 90 pounds of zinc per daypasses on with the con- The volume of gas to be handled by'thegas-cleaning apparatus is, at the temperature and pressure (vacuum)conditions obtaining in the apparatus, of the order of '500 cubic feetper minute. On this basis the dust load is 0.875 grain per cubic foot(2000 milligrams per cubic meter). At the pumping sitation after the gashas been returned to atmospheric pressure, the flow rate is about 200cubic feet per minute, with a corresponding dust loading of 2.19 grainsper cubic foot (5000 milligrams per cubic meter). This is entirely toolarge an amount of dirt to be handled by the pumping equipment andlines; in matter of fact, pipe line and pump valve obstructions havecccurred even with dust loads less than a tenth Thorough and completegas cleaning is necessary before condenser gases can be handled in apractical and commercial manner.

The removal of zinc fume or blue powder from carbon monoxide gaspresents problems not ordinarily encountered in gas-cleaning practice.Among the special difficulties posed are extreme neness of particle zincmetal particles; ready iniiammability` and explosive properties of zincdust when exposed to moisture and air; inflammability and explosiveproperties of carbon monoxide when mixed with more than a small amount(L1-5%) of oxygen; and tendency of any precipitated material to adheretenaciously to surfaces in such a manner as to lead to rapid obstructionof the cleaning appa' ratus.

With the present invention, cleaning eiciencies of the order of 99% areobtained, and, moreover, this cleaning is obtained using a minimumamount of water. Inasmuch as the water used in dust removal devicesoften is passed to settling ponds, thickeners, or other separationdevices wherein the solids may be recovered and the water recirculated,it is desirable to keep the amount of water used at a minimum so thatnot too great an investment will have to be madein settling basins orequivalent separating and recovery devices. i

Furthermore, this invention permits a continuous gas-cleaning operationin contradistinction to older methods wherein certain cyclic operationsare inherent due to necessity for bag shaking, filter cleaning, or thelike. Finally, the practice of the present invention does not require aslarge a capital investment in cleaning equipment as formerly employedmethods.

Typically the apparatus of the present invention includes a conduitcomprising a series of tubes having one or more abrupt changes indirection. Gas to be cleaned is passed through the conduit. Steam orother suitable vapor is admitted to the gas streamat a convenient pointin the conduit, preferably near the inlet end thereof. 'I'he gas thenpasses into the rst eductor or tube of the series. A fine spray of wateror other suitable liquid is introduced at the head or inlet end of theeductor. The tail of the eductor terminates in the side opening of thefollowing eductor, the head of which likewise contains means forintroducing iiuid in spray form. In this manner as many eductors asnecessary may be united to provide a gas conduit inwhich the gas streamis subjected to high velocity and vigorous turbulence by the liquidsprays and condensing vapor. tail of the final eductor leads to a gasdisengagement chamber where means are provided to separate the liquidfrom the gas, preferably by decreasing the linear gas velocity to a lowvalue by making the chamber of relatively large cross-section. Suitablylocated baflles also assist in precipitating by impingement mechanicallyentrained liquid. The separated liquid flows either to waste, if theliquid or solids are not of value, or to settling ponds, thickeners, orother suitable separating devices if the liquid or solids or both havevalue suilicient to warrant their recovery. The clean gas, for example,carbon monoxide, is compressed and made available for such purposes asmay be required.

The method of precipitating suspended particles from a stream of gasbearing the samein accordance with the principles of this inventionincludes introducing a condensible vapor, such as size; hydrophobicnature of Thesteam, into the gas stream, injecting a jet of liquid, forexample, water, into the gas stream to condense the vapor, toprecipitate substantially all the suspended particles and to acceleratethe flow of the gas stream, and separating the liquid from the gas, forexample, by substantially reducing the velocity of the gas stream or bycausing it to impinge upon a surface, cr by combined action of the two.

A still further object of this invention is to provide a process forcleaning gases bearing extremely finely divided solid material.

The objects and advantages of the present invention will be apparentfrom the following description-taken in connection with the accompanyingdrawing in which:

Fig. lis a front elevation of one form of apparatus embodying theprinciples of the invention;

Fig. 2 is a side elevation thereof; and

Fig. 3 is a plan View of the same.

- A preliminary gas Washer is indicated' at l. This gas washer is ofconventional design known in the art. It preferably is a shockprecipitator in which the gas from the zinc condensers is subjected to ashower of cold water and subsequently bubbled through water for thepurpose of precipitating the major portion of its burden of solidmaterial. ,The inlet 2 and an outlet 3 for drawing off the partiallycleaned gas. The outlet 3 also comprises an inlet for the secondarystage of the cleaning device. In this inlet, steam is injected throughpipe 4 and mingled with the gas. The valve 5 is providedto regulate theflow of gas through the apparatus. From this valve the gas passes to aneductor 6 which aids in circulating the gas through the system. A jet ofwater is injected into the eductor through pipe l for actuating thesame. 'I'he purpose of this jet is not only to aid in increasing theturbulence of flow of the gas, but also to assist in condensing thesteam which has been injected into the gas stream. Condensation of thesteam and intimate mixture o1' the gas with the water jet serves toprecipitate the finely divided solid material from the gas stream.

It has Ibeen found that, in general, a single eductor is not suilicientto completely precipitate all of the solid burden from the gas.Accordingly, in the apparatus shown, four eductors have been provided,each one actuated by a jet of water and each drawing on the precedingeductor. A larger or smaller number of eductors may be used in series asrequired. In the drawing, eductor 8 is arranged at right angles toeductor S and eductor 9 is positioned at right angles to'eductor 8 toform a zig-zag conduit. Waterjets are placed at each 'bend of theconduit. It has been found that four eductors in series, as illustrated,are sufficient for most purposes.

From the last eductor i0, the gas is forced into a chamber Il for theremoval of substantially all the water. The chamber is provided withsuitable baiiles I and is considerably larger in cross-section than theeductors so that the gas velocity is substantially reduced therein. Thestream of gas is diverted by the ybaiiles so that the condensed Watervapor is largely removed by impingement. The cleaned gas issues from thechamber Il through pipe i 3 and may be used or discarded as desired.

An outlet I4 is situated in the bottom of the chamber for the removal ofwater bearing precipitated particles. This slurry preferably is passedto settling basins where the solids are repreliminary gas washer has anI moved. The water may be recirculated through the apparatus.

Clean-out holes l are provided in the side of the chamber to give readyaccess to the interior thereof for cleaning`purposes. In actual practiceexperience shows that the baffles are substantially self-cleaning, andthe clean-out doors are opened only on unusual and rare occasions.

Preferably the zig-zag arrangement of eductors is positioned so that thewater jets are directed generally downwardly to allow the conduit todrain readily. However, it is not essential to so position the eductors,and any 'otherI suitable arrangement may be used.

Conduit l5, which carries the gas from the iinal eductor to the chamberIl, preferably extends through the wall of the chamber as shown and isoffset to the axis of the chamber to impart a rotary. movement to thegas in the chamber. This rotary movement results in considerableturbulence in the body of gas4 in the chamber and increases theefficiency thereof, due to the greater opportunity for impingement ofdrops of water against interior surfaces.

Although the conduits shown are of uniform cross-section, an alternativeconstruction may be desirable in some situations. In this constructionthe eductors are fashioned in the form of Venturi tubes. By thisconstruction better utilization of the velocity head of the lwaterstream in assisting the gas ow may be realized. However, this type ofconstruction is somewhat more expensive than simple pipes, hence thelatter are preferred for those situations where the gas is drawn throughthe apparatus by large capacity vacuum pumps, as in the zinc furnaceexample cited above. For low vacuum or pressure applications, theVenturi shape eduetor may in some situations be more economical.

In operation, gas, for example, from zinc condensers enters thepreliminary gas washer at 2 where it is relieved of a large portion ofits blue powder. The partially cleaned gas then passes through pipe 3where steam Thereafter it is drawn through theseries of eductors wheresubstantially all the residual blue powder is removed. From theeductors, the gas and liquid pass to the baille chamber H for theseparation of the liquid and gas. The cleaned gas issues through pipeing the removed solids is drained off through 'pipe I4.

The gas-cleaning efficiency of the device for very fine dusts isremarkable. With 0.22 micron diameter ,zinc dust gas clearances of havebeen obtained in practice even when treating gases the initialdustcontents of which are already low.

The operation of the present invention is characterized by substantiallycompleteI removal of even the very fine dust. The exact reason for theunusually superior performance of this gascleaning device is not exactlyunderstood. Without wishing to limit ourselves to .this theory, it issupposed that the dust particles serve as condensation nuclei for thesteam condensed by the water sprays. By this action the dust particle istrapped in a water droplet of much larger volume and mass than the dustparticle itself. By virtue of the enhanced mass and volume,'the particleand its surrounding droplet are easily washed down by the water sprays.While this mechanism undoubtedly occurs, it is not believed that itaccounts entirely for the highly effective dust removal obtained. It isprobable that the high I3, while the liquid containf over 99% isinjected into it.

'It is believed, from eductors of different diameters, that the mean Ydegree of turbulence and impingement action in combination with thewater droplets and condensing steam, together with a possibleilocculating action of the steam on the dust, are responsible for 4theefficacious removal of solid particles from the gas. It is also probablethat a wall action is involved and that an advantageous feature of theinvention is the high ratio of wall area. to gas volume provided by theinvention. experimental observations on path or average distance thatthe particles must travel before contacting a surface (wall) is animportant consideration in relation to dust re-l moval eiliciency. Ithas been found that decrease in gas velocity can be compensated for inpart by decrease in diameter of the conduit or eductor. Variation ofdiameter of the eductor influences the dust removing ability more thanvariation in length of the eductor.

In practice, an assembly in series of four eductors of 4-inch pipe. 25inches in length, has been found to effectively de-dust Zinc condensergas at a flow rate of 500 cubic feet per minute. The steam consumptionis 0.2 pound per minute; the water consumption (4 sprays) is 6 gallonsper minute. Devices of this type have replaced scrub towers consumingupward of gallons of water per minute.

A remarkable feature of the performance of the cleaner is -that highcleaning efficiencies are realized even though the gas is not heavilyladen with dust. In other words, not only the relative cleaning, but theabsolute cleaning as well is satisfactory. By the introduction of theinvention described herein to' commercial use in zinc production,significant savings, both directly in reduced operating and maintenancecosts, and indirectly through accompanying improvements in uniformityand stabilization of operations, have been realized.

It is possible to operate the gas cleaner without introduction of acondensible vapor. Even without the use of steam, considerablecleaningis realized. `But theuse of a condensible vaporin'- Iconjunction with the liquid stream and associated apparatus results in ahigher order of dust removal.

We claim:

l. A method for removing blue powder from zinc condenser gases whichcomprises passing astream of the gas in contact with a liquid to coolthe same and to precipitate a portion of the blue powder from the gas,introducing'a condensible vapor into the gas stream, injecting a jet ofliquid into the gas stream concurrently therewith to condense the vapor,all residual blue powder and to increase the turbulence of flow of thegas stream, and separating the liquid from the gas.

2. A method for removing blue powder from zinc condenser gases whichcomprises passing a stream of the'gas in contact with a curtain ofliquid to cool the same and to precipitate a portion of the blue powderfrom the gas, introducing a condensible vapor into the gas stream,injecting a jet of liquid into the gas stream concurrently therewith tocondense the vaporY to precipitate substantially all residual bluepowder and to increase the turbulence of flow of the gas stream,thereafter decreasing the velocity of the gas to a value sufficientlylow to permit substantially all entrained liquid particles to settlefrom the gas, and separating the `liquid from the gas. l

3. A method for removing blue powder from to precipitate substantiallyimpinge upon a surface all entrained moisture from zinc condenser gaseswhich comprises stream of the gas in contact with a liquid to cool thesame and to precipitate a porpassing a tion of the blue powder from thegasintroduc,

ing a condensible vapor intothe gas stream, injecting a jet of liquidinto the gas stream concurrently therewith to condense the vapor, toprecipitate substantially all residual blue powder and to;l increase theturbulence of flow of the gas stream, thereafter causing said streamkofgas to to remove substantially arating the liquid from the gas.

4. A method for removing blue powder from zinc condenser gases whichcomprises passing a stream of the gas in contact with a curtain ofcurtain of` the gas, and sep- 5.l A method for removing suspendedparticles from a'gas whichicomprises passing a stream of lall residualsuspended the gas in contact with a liquid to cool the same and toprecipitate 'a portion of the suspended particles from the gas,introducing a condensible vapor into the gas stream, injecting a jet ofliquid into the gas stream concurrently therewith to condense the vapor,to precipitate substantially particles and to increase the turbulence offlow of the gas stream, and separating the liquid from the gas.

6. A method for removing suspended particles from a gaswhich comprisespassing a stream .of the gas in contact with a curtain of water to coolthe same and to precipitate a portion of the sus, pended particles fromthe gas, introducing steam into the gas stream,.injecting a jet of waterinto the gas stream concurrently therewith to condense the steam, toprecipitate substantially all residual suspended particles and toincrease the turbulence of flow of the gas stream, and sep-.

arating the water from the gas.

i CARLETON C. LONG. GEORGE E. DEELEY.

